Change saturation of rendered object without changing the background Image
You can use an ID mask to do this.

Select the object you wish to color, go into object settings and change the pass index to 1.


Now go into the layer properties menu and enable "Object index" under passes.



Hit render. Now go into the compositor and add a viewer node so you can see what you are doing. You can use the "id mask" node as a mask for the object. Here I have setup a simple node network that will achieve what you want: